Most telecommunications carriers employ optical fibers in place of copper wires to carry telecommunications traffic. As compared to copper wires, optical fibers offer several advantages. Optical fibers possess much greater bandwidth in contrast to copper wires. Thus, a single optical fiber can carry many more voice conversations than a copper wire pair. Additionally, optical fibers are immune to electrical interference. Cross talk between two adjacent optical fibers within an optical fiber cable is nonexistent, whereas cross-talk between adjacent copper wires with the same cable can and does occur, resulting in signal deterioration.
For cosmetic reasons, as well as to provide protection against the elements, telecommunications carriers typically bury underground the optical fiber cables employed to carry long haul traffic. Unfortunately, burial does not render such optical fiber cables completely invulnerable to damage. Occasionally, a contractor excavating along an optical fiber cable right-of-way will inadvertently sever the cable. Since most fiber optic cables carry large volumes of telecommunications traffic, a severed optical fiber cable creates a major service disruption. For that reason, telecommunications carriers take great care to monitor their fiber optic cables to detect potential harm in an effort to avoid cable damage.
Various techniques exist for monitoring buried fiber optic cables. Once such technique is disclosed in U.S. Pat. No. 4,904,050, issued on Feb. 27, 1990, in the names of Lawrence Dunn et al. (herein incorporated by reference). The Dunn et al. '050 patent discloses the desirability of detecting intrusion on an optical fiber by an interferometric arrangement, whereby a pair of optical signal sub-signals, derived by splitting a single optical signal, are injected into opposite ends of the fiber via a coupler. In this way, each optical sub-signal injected into a fiber end emanates from the opposite fiber end. The optical sub-signals emanating from the fiber ends are recombined at the splitter for input to a detector that measures the phase difference between the signals as a detectable pattern. If an intrusion has occurred, the pattern detected by the detector will differ from the pattern detected under quiescent conditions (no intrusion).
U.S. Pat. No. 5,778,114, issued on Jul. 7, 1998, in the names of Hossein Eslambolchi and John Huffman, and assigned to AT&T, (incorporated by reference herein) describes and claims an fiber intrusion detection system that includes an optical splitter for splitting an optical signal into sub-signals for injection into opposite ends of fiber. The signals emanating from the opposite fiber ends are recombined at the splitter for receipt at a detector that measures the phase difference between the optical sub-signals. A processor compares the phase difference measured by the detector to phase difference measurements associated with different types of threats. By matching the actual phase difference to the phase difference measurement associated with a particular type of intrusion, the processor can thus identify the nature of the intrusion.
Neither the Dunn et al. '050 patent, nor the Eslambolchi et al. '114 patent provide any mechanism for determining the optical distance of the intrusion along the fiber from a central facility. Traditionally, the optical distance of an intrusion along the fiber is measured by the use of an Optical Time Domain Reflectometer (OTDR) that injects an optical signal into one end of a fiber in the cable for propagation therealong. The signal injected into the fiber will reflect back from a fault, such as a break in the fiber. By measuring the difference between transmission of the forward signal, and receipt of the reflected signal, of reflection, the OTDR can determine the optical distance to the fault. The optical distance to the fault often does not correspond to the sheath (route) distance along the cable because the cable typically contains loops at different splice points, thus making the optical distance longer. For that reason, a technician must rely on a cable installation drawing to correlate the measured optical fault distance to the sheath distance. Often, cable installation drawings may not accurately reflect loop lengths, thus yielding an inaccurate measure of the optical fault distance.
Thus, there is a need for a technique for obtaining a more accurate optical fault distance